Inter-Specific and Intra-Specific Competition of Two Sympatrically Breeding Seabirds, Chinstrap and Gentoo Penguins, at Two Neighboring Colonies

SIMPLE SUMMARY: Under limited resources, different species may coexist by segregating their food items and foraging time, but neighboring colonies within the same species may have highly overlapping foraging habits. Thus, it is hypothesized that intra-specific competition is more intense than the inter-specific competition. To test this hypothesis, we chose sympatrically breeding Chinstrap and Gentoo penguins at two neighboring colonies and tracked their foraging space and dive depths during chick-rearing. Here we report that there was a larger overlap in the foraging space between the two species than within each species, whereas there was lower inter-specific isotopic niche overlap than intra-specific isotopic niche overlap. Despite the low intra-specific spatial overlap, diets of conspecifics remained similar, resulting in the higher isotopic niche overlaps. Our results suggest that intra-specific competition is higher than inter-specific competition due to a lack of niche partitioning. ABSTRACT: Theory predicts that sympatric predators compete for food under conditions of limited resources. Competition would occur even within the same species, between neighboring populations, because of overlapping foraging habits. Thus, neighboring populations of the same species are hypothesized to face strong competition. To test the hypothesis that intra-specific competition is more intense than inter-specific competition owing to a lack of niche partitioning, we estimated the foraging area and diving depths of two colonial seabird species at two neighboring colonies. Using GPS and time-depth recorders, we tracked foraging space use of sympatric breeding Chinstrap and Gentoo penguins at Ardley Island (AI) and Narębski Point (NP) at King George Island, Antarctica. GPS tracks showed that there was a larger overlap in the foraging areas between the two species than within each species. In dive parameters, Gentoo penguins performed deeper and longer dives than Chinstrap penguins at the same colonies. At the colony level, Gentoo penguins from NP undertook deeper and longer dives than those at AI, whereas Chinstrap penguins did not show such intra-specific differences in dives. Stable isotope analysis of δ(13)C and δ(15)N isotopes in blood demonstrated both inter- and intra-specific differences. Both species of penguin at AI exhibited higher δ(13)C and δ(15)N values than those at NP, and in both locations, Gentoo penguins had higher δ(13)C and lower δ(15)N values than Chinstrap penguins. Isotopic niches showed that there were lower inter-specific overlaps than intra-specific overlaps. This suggests that, despite the low intra-specific spatial overlap, diets of conspecifics from different colonies remained more similar, resulting in the higher isotopic niche overlaps. Collectively, our results support the hypothesis that intra-specific competition is higher than inter-specific competition, leading to spatial segregation of the neighboring populations of the same species.